The invention relates generally to an X-ray inspection system and method, and more particularly to an X-ray inspection system and method for screening.
Many applications use X-ray diffraction to identify the crystal structure and composition of unknown objects spatially distributed along an incident X-ray beam. For example, some airport baggage screening systems use X-ray diffraction (XRD) to identify explosive threats in scanned baggage. When such analyses need to be performed quickly, the standard approach has been to employ energy dispersive X-ray diffraction (EDXRD) with expensive, energy-sensitive, liquid nitrogen (usually) cooled, single pixel, line, or array of line detectors. Frequently these systems employ X-ray sources that generate a divergent, polychromatic, X-ray beam that needs to be collimated and combined with a detector collimator in order to probe a localized volume of space. The size of the localized volume depends on the degree of collimation provided by the collimators. The major problem, therefore, becomes one of measurement time versus spatial resolution. If two collimators are used to provide a reasonable degree of collimation, i.e., good spatial resolution, the photon flux drops dramatically, requiring long measurement times to obtain good counting statistics.
In the case of EDXRD-based explosive detection systems (EDS), for example, both good spatial resolution and fast measurement times are needed. When both source and detector collimators are used, the X-ray intensity is typically reduced by more than 99.99%, leading to low-intensity signals that increase the difficulty of correct explosive threat identification. To compensate for these low signals and address the high false positives and commensurate slow scan rate, EDS manufacturers use single-photon counting, energy-sensitive, single pixel, line, or array of line detectors, e.g., liquid nitrogen cooled, high purity Ge (HPGe, high performance Ge) detectors with small X-ray sensitive areas. An inherent problem with these single-photon counting detectors is their inability to distinguish between two photons of equal energy incident upon the detector simultaneously and a single photon with twice the energy. Mischaracterization of the energy of the incident photon will lead to reduced sensitivity and specificity. Additionally, the detector dark currents (electrical signals recorded with no photons impinging on the detector), which are different for each detector element, are temperature-dependent. At liquid nitrogen temperatures, the dark currents are negligible, but increase non-linearly with increasing temperature. Thus, as the liquid nitrogen boils off and the detector element temperature changes, the dark current increase will not be uniform, degrading the signal across the detector elements differently, which may affect the accuracy of threat detection. The dark current also changes with time as the detectors are exposed to high-energy X rays, possibly affecting the accuracy of threat detection also. Furthermore, the HPGe detectors typically have count rate limitations; they saturate quickly when the diffracting volume contains strongly diffracting crystals, which in turn increases the difficulty of accurate composition identification. Moreover, the operationally acceptable baggage scan rates are such that the counting statistics in the diffracted signal are very low, leading to classification errors.
One solution is to develop more powerful X-ray sources that provide a higher photon flux density. However, most current X-ray sources are already operating near the limit where the target melts, which shortens their lifetime, making source maintenance a concern in explosives detection systems.
Thus, it would be desirable to have an X-ray inspection and detection system having higher accuracy, higher speed, and lower maintenance costs.